In a gasifier used for production of combustible gas (fuel gas) from solid fuels--in particular peat and biomass fuels--the gasifier power is dependent on the gasification air flow (i.e. the primary air flow). The relationship between the gasifier power and the gasification air flow is valid independent of the procedure for the fuel input to the gasifier, except for the case when the fuel flow is limiting the gasifier power. Therefore, this relationship is valid for both fluidized beds and fixed fuel beds including fluid bed boilers, conventional fixed bed boilers and the more specific co-current gasifiers and counter-current gasifiers. Thus, all of these different boilers used for burning solid fuels form a combination of a gasifier (i.e. gasproducer) and a gas burner. The more specific difference between the different boiler types is the appearance of the combination of the gas producer and the gas burner. Independently of the appearance of the combination, the two different process stages anyhow do exist.
Boilers and gasifiers for solid fuels are usually designed for a very narrow interval with regard to fuel quality. This means that combustion equipment is constructed for fuels having high moisture contents, for comparatively dry fuel and finally for very dry fuel, i.e. usually pelleted fuel. If a boiler plant designed for using dry fuel or very dry fuel is used for burning wet fuel, i.e. fuel with a high moisture content, the result regarding combustion quality is usually very poor. Contrary, if a boiler designed for burning fuel with a high moisture content is used for burning dry fuel, insuperable problems usually will arise in connection with the fuel ash handling. Although gasifiers (i.e. gas producers) exist operating almost without problems when using both wet and dry fuel, a remaining problem is that the gasification air flow (i.e. the primary air flow) often gets too high when the fuel quality is decreased. This excessive air flow then will cool down the gasification process, which causes a gasification power reduction because of the strong relationship between gasification process temperature and gasification power.